[llvm] [AMDGPU][NFC] Remove direct access to FeatureKV (PR #206231)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

Unrelated: this file doesn't follow
https://llvm.org/docs/CodingStandards.html#restrict-visibility

https://github.com/llvm/llvm-project/pull/206231